1. Field of the Invention
The present invention relates to a conveying device for a scanning module, and more particularly to a conveying device having a planetary gear transmission to adjust the resolution (DPI, dots per inch) of a scanner.
2. The Related Art
In image scanning field, the quality of the image is required better, so the resolution (dots per inch) of the image is required higher and higher.
A conveying device includes a planetary gear transmission a belt and a scanning device. In order to improve the quality of the image, a spur gear and a speed-change chest are used in the gear transmission. The speed-change chest comprises a gear speed-change module, the gear speed-change module consists of a plurality of gears having different numbers of teeth. Different gear results in different resolution, so when a certain resolution is desired, a gear having proper teeth is chosen to clench the teeth of a belt gear, the belt gear drives the belt move, then the scanning device connected to the belt moves along with the belt to scan a document.
When a certain resolution is desired, a gear having proper teeth is chosen, however, the number of the gears is limited, as a result, the resolution is limited. Meanwhile, one stepping motor is used in the gear transmission to control the rotational speed. On one hand, when the motor rotates too slowly the motor shakes, so the quality of the scanning is not good, on the other hand, when the motor rotates too quickly the motor is unsteady and the resolution is not good yet. Moreover, the volume of the gear transmission is large and the structure is complex, as a result, the rotation is not stable and the quality of the image is not good.